I noticed that lots of people have this problem in the Internet, but I haven't found any good solution. I've had this problem for quite sometime and I still don't know how to fix it. I have updated my Drivers to 260.10 and tried older drivers too but they don't work.
GTX295 is Coop SLI which means it's one card with 2 GPUs.

There were two revisions of the GTX295. The first was a card that had two PCBs with one GPU on each PCB and had fixed connections running between both PCBs to enable SLI. The second version put both GPUs on the same PCB.

If you have the early version of the GTX295, the SLI connection(s) between both PCBs may have lost connection due to thermal stress on the card. You could attempt to de-sandwich the PCBs and put them back together to see if it's a connection issue.

If that doesn't work, or you have the later version with two GPUs on the same PCB, then one of the GPUs may have suffered from a failed BGA soldering connection. Properly fixing a bad BGA joint is really annoying, but can be done.